This invention relates to the pyrometallurgical treatment of finely divided ores, concentrates, residues, mattes, slags, and like materials, more particularly to methods and apparatus for use in such treatment.
2. Description of the Prior Art
The extraction of elemental metals from the materials in which they occur, both, for example, naturally in the form of ores and man-made in the forms of slags and residues, is undergoing dramatic change brought on by the ever-increasing cost of energy and a heightened concern for the environment. The chemistry of such extraction processes is old and well-known; basically, the problem is to separate the elemental metal from other chemical elements or compounds with which it is bound such as, for example, oxygen (oxides) and sulfur (sulfides). The traditional solution has been to apply energy in whatever form and amount are needed to effect the extraction and without much regard to the nature of the by-products produced in such extraction processes or their impact on the environment. This approach can no longer be tolerated from an economic, environmental or legal standpoint. The new emphasis in metal extraction processes thus centers around capital minimization, energy efficiency, product yield, and by-product recovery, and specifically including solid, molten, and gaseous by-products and effluent control.
A notable example of the foregoing problem is in the production of zinc. Zinc in the elemental state is not known to occur in nature and, therefore, it must be extracted from zinciferrous materials by selected processes. Those processes that have been or are presently being used commercially may be classified as (a) horizontal retort, (b) vertical retort, (c) electrothermic, (d) blast furnace, and (e) electrolytic.
In general, the thermal processes, namely, (a) through (d) above, are based on the principle of carbothermic reduction for extracting zinc from zinc-bearing materials. Carbon monoxide gas (CO) or solid carbon is the primary reducing agent in these processes, but zinc oxide can be reduced to zinc metal only at temperatures usually well above the boiling point of elemental zinc which is 907.degree. C. (1664.6.degree. F.). Large amounts of energy are expended in achieving and maintaining such temperatures.
The traditional thermal processes suffer from several shortcomings including the need to prepare a hard, agglomerated feed to withstand furnacing; slow reaction rates, requiring long residence times usually in one very large or numerous small reactor unit(s); the use of large quantities and expensive forms of energy such as lump coke, charcoal, electricity, and natural gas, in some cases due to indirect heating of the charge; and high captial and operating costs per unit of product, thereby requiring relatively large plant capacities to be economical. Likewise, the electrolytic process, while more technically advanced than most of the thermal processes, still suffers from high capital, energy and operating costs.
In recent years, a number of efforts have been focused worldwide on overcoming some of the inherent shortfalls of traditional thermal smelting. These efforts have resulted in several methods of recovering metal values from finely divided ores, concentrates, calcines, and slags by flash smelting in reactor-type vessels. Flash smelting does not require preagglomeration of metal-bearing feeds; has a high volumetric rate of throughput (or short residence time); can use cheaper forms of thermal energy such as fine coke, coal, charcoal and waste carbon, and/or sulfide fuel; can be easily automated; and often can be oxygen-blown, decreasing the off-gas handling volume and problems. This method therefore generally has better energy utilization and lower operating costs than the traditional methods. Furthermore such reactors generally have lower capital requirements and the size of the unit can be much smaller than traditional furnace installations of like capacity.
As examples of the just-mentioned process, see Derham U.S. Pat. No. 3,271,134 (zinc calcine); Blaskowki U.S. Pat. No. 3,607,224 (iron ore); "Flash Smelting of Lead Concentrates," Bryk et al., J. of Metals, December 1966, 1298-1302; "The KIVCET Cyclone Smelting Process for Impure Copper Concentrates," Melcher et al., J. of Metals, July 1976, 4-8; "The Boliden INRED Process for Smelting Reduction of Fine-Grained Iron Oxides and Concentrates," Elvander et al., Third Int. Iron & Steel Cong. Proc., April 1978, Chicago, IL, 195-200; and "The Chemistry of the ELRED Process" (iron ore), Bengtsson et al., I & SM, October 1961, 30-34. These processes have met with varying degrees of success, but all are believed to have limitations brought about by (a) the complexity and expense of the associated equipment needed to practice them, (b) an inability to process a variety of feed materials, and/or (c) difficulty in controlling and stabilizing process parameters. Some of the processes have never operated commercially. In general, most commercial flash smelting units are limited to one-step oxidation of sulfidic feeds, while other processes for oxide feeds involve more than one step or reactor.
The invention presented here addresses some of the inherent shortfalls of current flash-smelting technology.